Acoustic inspection is a recognized technology for investigating the down-hole environment in well bores, in both open- and cased-hole environments. Acoustics have been used to investigate cement bond quality for decades (see, e.g., U.S. Pat. No. 4,255,798), but in most applications the frequency range used is on the order of kHz. Higher frequencies allow for superior resolution, but suffer from a number of design constraints that restrict their application in down-hole applications. In view of these constraints, frequencies in the MHz range have only rarely been employed.
The downhole environment presents significant technical challenges for acoustic sensors and their supporting electronics. Modern well bores are routinely 15 kilofeet in depth, and some deeper wells are over 30 kilofeet. These figures reflect typical true depths; the length of the well bore is often significantly greater. Due to the use of directional drilling, for example to exploit a larger area in a field from a single production platform, the well bores are typically not vertical through at least a substantial portion of their length. Thus, the downhole environment presents pressures frequently in excess of 20-30 kPSI, and temperatures frequently in excess of 175-225° C. These conditions make it important to detect even small defects in the casing that lines the well bore. One object of embodiments disclosed herein is to detect pits, gouges, etc., in metallic casing on the order of 1 mm. Another object is to detect defects in the quality of the cement bond behind metallic casing on the order of 1 cm.
The well bore is protected from collapse by approximately maintaining hydrostatic pressure in the well bore with the surrounding environment. The weighted fluid that is typically used to achieve this is generally referred to merely as “mud,” but it is actually a carefully engineered fluid that often costs more per barrel than the hydrocarbons that are typically the object of the well. Depending on the demands of the particular drilling project, mud may weigh more than 25 lbs/gals.
What is needed, therefore, is a means to acoustically inspect casing and cement bonds that provides high resolution, yet still operates in high density fluids. Embodiments disclosed herein meet this need.